Electron beams of narrow dimensions and high energies are commonly employed to create patterns on semiconductor wafers and masking materials by altering the chemical nature of a resist overlayer. In electron sensitive resist polymethylmethacrylate (PMMA) bonds are broken, decreasing the average molecular weight of the long chain molecules. This increases the solubility of the resist in a suitable developer, allowing the exposed pattern to be indented into the resist. In further processing steps, the pattern is transferred by various techniques to the substrate material. An important limitation on the density of the exposed pattern, and in some respects the individual line widths, is the back-scattering of electrons from the substrate, causing exposure of the resist several microns away from the area of the primary exposure. This phenomenon, causing the proximity effect, reduces the definition of the exposed pattern.
As lines are placed closer together, the total dose increases between the primary exposed regions due to the proximity effect, thus leading to a decreased differential in exposure. The requirements of the developer to distinguish between the difference in doses becomes more stringent if the pattern is to be resolved. Ideally, a developer will develop (i.e. remove) all resist which has been exposed to a dose above a certain threshold and none of the resist which has been exposed below that threshold. In practice, a range of exposures results in partial development of the resist.
The general process of high resolution electron beam lithography with a PMMA resist is described by H. G. Craighead, "10-nm Resolution electron beam lithography", J. Appl. Phys. 55:4430-4435 (1984). See also H. G. Craighead, "Ultra-High Resolution Electron-Beam Lithography", J. Electron Microscopy Technique", 2: 147-155 (1985); S. Mackie et al., "Materials and Processes for Nanometer Lithography", Solid State Technology/August 1985, 117-122; and K. Harada et al., "Detailed Contrast .gamma.(-Value) Measurements of Positive Electron Resists", J. Electrochem. Soc., Nov. 1982, 2576-2580.
Three chemical solutions are commonly employed as electron-resist developers. These are:.(1) methylethylketone:ethanol in a ratio of 23.5:76.5 (v/v), referred to as "MEK"; (2) 2-ethoxyethanol (Cellosolve): methanol in a ratio of 3:7 (v/v), referred to as CS; and (3) methylisobutylketone: 2-propanol in a ratio of 25:75 (v/v), referred to as MIBK. Of these, MEK is the poorest for high resolution applications and its use is restricted to applications which do not require high contrast.
We have discovered that the addition of small amounts of MEK to either CS or MIBK alone or in a mixture of these developers unexpectedly and drastically improves contrast and reduces the required dose, thereby providing a significantly improved developer for PMMA resist applications in high resolution electron beam lithography, and hence improved processes for fabricating integrated circuits.